ADS-B explained simply: Out, In, and what it changes for you

ADS-B (Automatic Dependent Surveillance – Broadcast) is an electronic surveillance standard used by commercial and general aviation. An equipped aircraft determines its position, most often via GNSS, and then broadcasts it automatically without any external interrogation being required — hence the terms "dependent" (dependent on onboard systems) and "broadcast".

The ADS-B Out function consists, for an aircraft, of broadcasting its identity, position, and flight data derived from its own onboard systems (altitude, speed, heading, among others depending on the equipment). It is this function that makes an aircraft detectable by surrounding ADS-B receivers — on the ground as well as in flight.

The ADS-B In function consists of receiving ADS-B transmissions broadcast by other aircraft. A device may have one of the two functions without the other: an ADS-B In receiver alone does not broadcast anything, and an ADS-B Out transmitter alone does not receive anything. These are two independent capabilities, often combined but not systematically.

ADS-B is widely used by commercial and general aviation, where its equipment is governed by regulatory mandates depending on the airspace and jurisdiction. Its use is also growing, more variably, in light aviation depending on the country and the aircraft categories concerned.

ADS-L is not a miniature or simplified version of ADS-B. These are two distinct technologies and two distinct ecosystems, with their own frequency bands, their own technical constraints, and their own target audiences. Confusing the two leads to false expectations about compatibility between devices.

ADS-B and FLARM are distinct protocols, developed separately and addressing different needs. Coexistence is possible on certain multi-protocol devices that integrate both, but this depends entirely on the installed hardware — it is not a general property of the two standards.

Mode S is a surveillance transponder standard, used to respond to interrogations from a ground-based secondary radar or a TCAS/ACAS collision avoidance system. It is not synonymous with ADS-B: in practice, the ADS-B Out function is often implemented via an extension of the Mode S signal (the extended squitter), but a Mode S transponder alone, without this extension enabled, does not broadcast the same data as an ADS-B Out transmitter. The two standards remain conceptually distinct.

Some receivers and variometers on the market include ADS-B In reception depending on the model and manufacturer. This capability cannot be generalized to all free flight devices: it must be verified case by case in the technical specifications of each instrument, via our catalogue.

This is the most important point to remember on this page. Receiving data (In function) does not mean transmitting it (Out function). A device that only has ADS-B In reception allows you to see certain surrounding aircraft, but does not make you visible to them in any way.

To be seen by aircraft that monitor ADS-B traffic, you need an active and properly configured ADS-B Out. A standard ADS-B In receiver does not natively receive other transmission protocols such as ADS-L Out: this requires an ADS-L receiver or a compatible multimode device, or, depending on the future FASST-CH architecture, a retransmission via ground-based infrastructure. A simple receiver, however capable, is never a substitute for a transmission function — nor does it make visible the protocols it cannot receive.